The disclosure relates generally to the field of borehole drilling using a pump to lift drilling fluid out of the borehole so as to maintain a selected borehole pressure. More specifically, the disclosure relates to mud return pumps and methods for connecting such pumps to a drilling riser.
FIG. 1 shows an example “mud lift” drilling system using a drilling fluid (“mud”) return pump when drilling from a drilling unit 1 comprising a derrick 6 above the surface 10 of a body of water. In construction of a sub-bottom borehole using the system in FIG. 1, a conductor pipe may first be driven into or jetted into formations below the water bottom 8. When drilling a borehole 15 from the drilling device, drilling fluid is pumped through a drill string 16 down to a drilling tool, usually including a drill bit (not shown). The drilling fluid serves several purposes, one of which is to transport drill cuttings out of the borehole 15. The drilling fluid flows back through an annular space (“annulus”) 30 between the borehole wall, the liner or surface casing 14. The annulus 30 is typically in fluid communication with a drilling riser 12 at a wellhead (not shown) proximate the water bottom 8. The riser 12 may extent to a drilling unit 1, where the drilling fluid is treated and conditioned before being pumped back down the drill string 16 into the borehole 15. In many cases, the drilling fluid in the drilling riser 12 and the annulus 30 will result in a head of pressure in the borehole 15 that is undesirable.
By placing a pump 20 in fluid communication with the interior of the liner 14 near the water bottom 8, or making a similar fluid connection to the interior of the drilling riser 12 at a selected elevation, which may be above the water bottom 8, the returning drilling fluid may be pumped out of the annulus 30 and up to the drilling unit 1. The annular volume in the riser 12 above the drilling fluid may be filled with a riser fluid that is of a different composition than the drilling fluid. Preferably, the density of the riser fluid is less than that of the drilling fluid.
The drilling fluid pressure at the water bottom 8 may be controlled from the drilling unit 1 by selecting the inlet pressure to the pump 20. The height H1 of the column of drilling fluid above the water bottom 8 depends on the selected inlet pressure of the pump 20, the density of the drilling fluid and the density of the riser fluid. The inlet pressure of the pump 20 is equal to: P=(H1γb+H2γs)*C, wherein γb=the density of the drilling fluid, H2=the height of the column of riser fluid, γs=the density of the riser fluid and C is a constant.
In order to prevent the drilling fluid pressure from exceeding an acceptable level (e.g., in the case of a pipe trip), the drilling riser 12 may be provided with a dump valve. A dump valve of this type may be set to open at a particular predetermined pressure for outflow of drilling fluid to the body of water.
The following describes a non-limiting example of a method and device illustrated in the accompanying drawings, in which, as noted above, FIG. 1 is a schematic view of a fixed drilling rig provided with a pump for the returning drilling fluid, the pump being coupled to the riser section near the seabed and the riser section or portion thereof being filled with a fluid of a different density than that of the drilling fluid.
Reference number 1 denotes a drilling unit comprising a support structure 2, a deck 4 and a derrick 6. The support structure 2 is placed on the water bottom 8 (or the support structure 2 may be affixed to flotation devices as is well known in the art) and projects above the surface 10 of the water. The riser section of the surface casing or liner 14 extends from the water bottom 8 up to the deck 4, while the liner 14 extends further down into the borehole 15. The drilling riser 12 may be provided with required well head valves such as a subsea blowout preventer assembly (“BOP”) 104. The BOP 104 may include various devices known in the art to close the borehole 15 hydraulically when the drill string 16 is in the borehole 15, or when there is no drill string present.
The drill string 16 projects from the deck 4 and down through the liner 14. A first pump pipe 17 is coupled to the riser section 12 near the water bottom 8 via a valve 18 and the opposite end portion of the pump pipe 17 is coupled to a pump 20 placed near the seabed 8. A second pump pipe 22 runs from the pump 20 up to a collection tank 24 for drilling fluid on the deck 4.
A tank 26 for a riser fluid communicates with the riser section 12 via a connecting pipe 28 at the deck 4. The connecting pipe 28 may have a volume flow meter (not shown). In some embodiments, the density of the riser fluid is less than that of the drilling fluid. The riser fluid may be a gas in which case the tank 26 and connecting pipe 28 can be omitted.
The power supply to the pump 20 may be via an electrical or hydraulic cable (not shown) from the drilling unit 1. The pump 20 may be electrically driven, or may be driven hydraulically by means of oil that is circulated back to the drilling unit 1 or by means of water that is dumped in the sea from the pump 20 power fluid discharge. The pressure at the inlet to the pump 20 is selected from the drilling unit 1.
The drilling fluid is pumped down through the drill string 16 in a manner that is known in the art, for example, using a mud pump 27 which lift mud from a storage tank 24 and discharges drilling fluid (“mud”) under pressure to the interior of the drill string 16. returning to the deck 4 through an annulus 30 between the liner or casing 14 (and the riser 12) and the drill string 16 through a return line 29. When the pump 20 is started, the drilling fluid is returned from the annulus 30 via the pump 20 to the storage tank 24 on the deck 4. Using such a system it is possible to obtain a significant reduction in the pressure of the drilling fluid in the borehole 15 and consequently a higher mud density may be used creating a different pressure gradient.
The riser 12 may include auxiliary fluid lines 100, 102 that may be in selective hydraulic communication with the borehole 15 below the BOP 100. Such lines may be known by names such as “choke line”, “booster line”, “kill line”, etc., depending on the use of the individual line 100, 102.